Applying building energy modeling

Whole building energy modeling techniques and software are crucial for designing code-compliant buildings. At the same time, this near-mandatory exercise introduces opportunities for engineers to add significant value to projects.

By Paul Erickson, LEED AP; and Bill Talbert, PE, LEED AP, Affiliated Engineers Inc. October 16, 2014

Learning objectives

  • Understand what building energy modeling is and why it is used.
  • Recognize how energy modeling tools and techniques can be used throughout the entire design process and the value they can provide.
  • Learn how to use energy modeling to maintain and improve building performance over the life of the building.

Driven by increasingly stringent building energy codes and certification programs, owners’ operating cost needs, institutional sustainability imperatives, and growing regulatory legislation, high-performance building design has become a fundamental component of the overall building design process. The high-performance building design process integrates building system design to optimize overall building energy and water use, operability, function, cost, and resilience over the intended life of the building.

With the desire to reduce carbon emissions accelerating the pace of energy code and standards development, prescriptive design of compliant building systems is more and more challenging. At the same time, participation in building certification programs that include energy-performance improvements over energy codes is on the rise.

To meet these needs, engineering-led building energy modeling provides design teams with a powerful tool for predicting how design characteristics will affect energy use and cost, assisting in the creation of integrated building systems that can exceed building performance targets associated with project program and budget goals. But energy modeling not only provides owners with the knowledge necessary to make informed decisions in satisfying energy code requirements, it also provides the data necessary to continue improvement of building operation over the life of the facility.

Code compliance

Ten years ago, energy modeling executed during the design phases of projects was generally still relegated to the role of documentation, whether for code compliance, the U.S. Green Building Council’s (USGBC) LEED rating system, or possibly a few lifecycle cost scenarios. Compared to today, there were very few modeling practitioners, the budget for such activity was often limited, and many of the tools were not nearly as user-friendly.

The rapid growth of the LEED rating system and other certification programs that require energy performance improvements over energy code has had a significant impact on the role of energy modeling in design, both directly and indirectly. The direct impact, attributable to the need or preference of many projects to complete modeling, has led to development of energy modeling talent within architecture, engineering, and construction (AEC) firms and has created a large contingent of firms that provide LEED, energy modeling, and related consulting services.

Indirect impacts are largely the result of heightened awareness of energy issues attributable to the USGBC’s efforts, manifested in the form of institutional policy or legislation. One example is the decision by many organizations to require that all new projects achieve particular energy savings targets as compared to code because their LEED projects were not necessarily achieving high scores in the energy savings credit. The University of California, the University of Michigan, and the State of North Carolina are such entities.

Energy modeling for code compliance

While some states were still using the 1989 version of ASHRAE Standard 90.1 as recently as 5 years ago, today most states have adopted recent versions of ASHRAE 90.1 (2013, 2010, or 2007) or the International Energy Conservation Code (IECC) (2012 or 2009). (See Figure 1.) This reflects the importance being placed on energy and energy performance in buildings and has in part been driven by federal legislation advancing the energy codes on a more regular basis. It also has placed a challenging requirement on project teams to demonstrate compliance. In the past many teams used a "prescriptive" path within these standards to demonstrate compliance (e.g., limit window area, use a certain level of insulation, select an air conditioning unit with a certain efficiency or better), but with performance requirements trending higher for code, many are forced to use a "performance" path to demonstrate compliance.

Energy modeling is used to support the performance path, allowing project teams to make trade-offs (e.g., perhaps the window area and fan horsepower don’t comply with code requirements, but improved glass and more efficient HVAC system types can provide counterbalance). The energy model for the prescriptive path takes all of these factors into account to demonstrate that the proposed building will perform as well or better than the code minimum building.

Mandatory energy targets, lifecycle cost

States, universities, the federal government, and other organizations across the country have been incorporating energy savings targets into their design standards, many using a target of 30% savings as compared to code (e.g., the federal government, the states of Iowa and North Carolina, the University of Michigan, the University of Texas, and Cornell University). The University of California’s Office of the President instituted a 20% better-than-code target. Regardless of the savings target, this requirement is being applied to all of the institution’s projects as part of its design standards. These and other entities are also applying more aggressive targets on individual projects. In many cases this takes the form of energy use intensity (EUI) targets and even net-zero energy. Cornell University’s approach is an example of considering percent energy savings reductions to develop EUI targets for various building types.

All of these have contributed to the increased demand for energy modeling. However, to evaluate the various possible pathways for achieving the targeted savings in the most cost-effective manner, energy modeling functions more as a complex, dynamic platform than as a simple verification tool. Cost effectiveness is paramount to implementing truly efficient design strategies; many organizations allow exemptions if reaching the target cannot be justified based on economics.

The private sector typically uses return on investment as the basis for decisions, often looking for returns that essentially provide payback in the 1- to 5-year range. Institutional clients tend to think in terms of total cost of ownership over a 20- to 30-year life. Lifecycle cost (LCC) is generally used to evaluate the economics. The Dept. of Energy’s BLCC tool lifecycle cost analysis (LCCA) is one of several that are freely available and it, like others, may be the required tool depending on the client.

Integrated design

Achieving energy targets is often no easy feat for many project teams, especially if design option testing doesn’t begin early. The need for engineers and architects to engage in design conversations much earlier and to better understand the interplay of design options and decisions on the part of both is what the American Institute of Architects, USGBC, and others meant by "integrated design" when they began to promote the concept well over a decade ago and continue to do so today in such publications as "An Architect’s Guide to Integrating Energy Modeling in the Design Process." Though these mandates do not hold design teams accountable for the full breadth of integrated design, they do drive it.

There is still a prevalent attitude that modeling should begin late in the schematic phase of a project when more details are known, so as to not consume too much fee with the modeling process. Modeling at this point can still serve to assist in achieving energy targets. More encouragingly, an awareness is growing that minimal additional investment can create a great opportunity to leverage the energy model even earlier (programming and concept phases), increasing the potential impact and value of using modeling to inform the design process. Figure 2 shows an example of how early testing of strategies served to identify architectural and engineering strategies for meeting the project energy mandate. The models and knowledge gained can then be used continuously throughout the design process.

Leveraging energy modeling techniques and software in design

The complex interaction of building systems, exterior climate, and occupant actions makes understanding building energy performance a design challenge for which energy modeling is particularly well suited. The building components and characteristics that determine energy use by the building need to be included in an energy model simulation (see Figure 3). Every decision made as a building design progresses from the conceptual stage through the completion of construction documents has potential impact on energy use. Waiting until the later stages of the design process limits the impact and benefit that energy modeling can provide. Understanding how much energy a building will use and how it compares to the maximum energy use allowed by an energy code or to a baseline building starts with conceptual development.

Programming, conceptual design phases

Space programming initiates conceptual design; building form and aesthetic decisions are made along with preliminary building load estimates for testing building system options. Employed at this stage, energy modeling quantifies the effect of building shape, orientation, and massing on daylighting potential and heating and cooling loads. Site and climate-related factors such as temperature, wind speed, and solar access are considered for direct impacts on system loads and types, and on natural ventilation opportunities. Other environmental factors, such as air quality and noise, also need to be considered and accounted for so proposed systems aren’t modeled in isolation from real-world design limitations.

As detailed building geometry and space descriptions are not available at this stage of the design process, the analysis methodology primarily needs to incorporate and account for overall building massing, general building and space usage patterns, and site-specific climate data. Simplified geometry and zoning based on the space types identified in programming allow the creation of energy models that are accurate enough to direct design decisions through comparisons of relative differences between scenarios and to allow quick exploration of multiple design strategies. Comparisons of relative differentials between design strategies are typically within 10% of the values estimated at later stages of the energy modeling process when final building geometry and zoning are available.

The goals of the conceptual phase analysis should focus on quantifying the impacts of massing, orientation, building envelope performance, and space program on building heating, cooling, and electrical system loads and overall energy performance as well as identifying building mechanical and lighting system energy savings strategies. Projects’ energy performance goals and requirements should be identified during predesign and concepts. It is not too early to begin comparing system types to understand how the proposed project performance compares to the code-compliant baseline and begin to understand energy and construction cost differences between options. Figure 4 provides an example of how the energy model can not only provide the numerical results, but also be helpful for visually articulating the differences between scenarios being evaluated.

Renewable energy strategies can also be assessed for their ability to offset energy supply needs, especially in the case of projects that are pursuing net-zero energy targets, or in climates and locations where solar and wind resources provide significant opportunities.

Detailed design phases

While project delivery methods may range from the traditional design-bid-build approach to design-build, construction management-led, and integrated project delivery, as the project proceeds past conceptual design, energy modeling continues to be a valuable tool for making design decisions, verifying code compliance, and measuring performance targets. With energy modeling initially adopted into project workflows largely due to LEED, analysis was likely initiated at the schematic design phase or later as project designers didn’t necessarily recognize the value and were hesitant to invest significant efforts during earlier phases when so many parameters were unknown. With the improvement of analysis tools, improved energy modeling workflows, and maturation of the analyst workforce, the value of earlier phase modeling is being recognized and more effectively integrated into project work plans.

While many design decisions on a project are yet to be made, those factors affecting energy use and cost are best resolved by the end of the schematic design phase using LCCA. Energy modeling in concert with construction cost estimating can be used to determine the options with the best value at a time in the design process when space planning for building systems and system type selection is occurring. Once design development and later phases have been reached, energy analysis may be best suited for value engineering (VE), equipment selection, and bid evaluation exercises. Although it often becomes increasingly difficult to significantly change design approaches due to the perception, real or perceived, that construction costs and design efforts will increase, energy analysis remains well-suited for design refinement. During the construction document and bidding phases, the energy model can be used for final code compliance documentation, building certification program documentation and further design verification or VE.

Benchmarking and energy analysis

Using benchmark data for energy use at similar facilities is a useful tool to provide context to project performance. Benchmark data is now more readily available through such Web-based tools as the Dept. of Energy Buildings Performance Database, EnergyIQ, and Energy Star’s Portfolio Manager that uses data from the Commercial Buildings Energy Consumption Survey (CBECS), the California Commercial End Use Survey (CEUS), as well as more user provided data. The state of Minnesota recently rolled out its B3 Benchmarking program, and many universities and large institutional clients are benchmarking their building portfolios. This data is very informative when developing energy models, as the information provides a comparison to actual building energy use for like facilities and climates. It serves as a quality control mechanism as well as a comparative performance metric for simulations.

Energy modeling has grown from manual calculation methods developed in the 1960s to complex computer modeling tools used today with the significant growth in building energy modeling driven by LEED and energy code compliance requirements. Currently, numerous software tools are available for energy modeling, ranging from such specialized component-based modeling tools as COMFEN and THERM, to such whole building analysis tools as EnergyPlus, eQUEST, Trace 700, and IES-VE, to specialized tools for renewable energy system analysis including System Advisor Model and RETScreen. The choice of energy model tool should be based on the purpose of the analysis and types of systems being analyzed, though the choice may be defined by the owner.

All software tools are not created equal, however. Owners, design teams, and energy analysts should all be aware of each software’s capabilities and limitations before selecting an analysis tool. Many current whole building analysis tools are suitable for quick conceptual phase analysis thanks to the development of increasingly user-friendly front-end graphical user interfaces that harness the power of the calculation engines on the back end. eQUEST is an interface that uses DOE2.2 as the calculation engine while Bentley’s AECOSim uses EnergyPlus as its calculation engine. Many tools offer methods to run multiple parametric simulations, which is also important for design phase analysis as there usually isn’t one single scenario or answer in the design process. In addition, multiple analysis tools may be necessary to adequately simulate complex or novel systems. Software such as TRNSYS is well suited to creating custom system configurations. Custom configuration also allows for integration with other tools in the design process, whether that be for daylighting, computational fluid dynamics, or integrated water modeling.

For energy code compliance, minimum modeling software requirements may be specified, as they are in ASHRAE 90.1 and the IECC. For instance, ASHRAE 90.1-2013 requires a whole building analysis tool capable of explicitly modeling 8760 hours/year, including hourly variations in occupancy, lighting power, miscellaneous equipment power, thermostat setpoints, and HVAC system operation, as well as thermal mass effects and part-load performance curves for mechanical equipment among other parameters. The whole building analysis tools mentioned above generally satisfy the software requirements specified in ASHRAE 90.1-2013.

In the past, the creation of the baseline compliant model was a completely manual process. Many current versions of software tools are offering some degree of automated baseline model generation ranging from simple library items with code compliant baseline characteristics to automatic selection of baseline characteristics based on user input for project location and applicable code. Fully automated baseline generation is not currently available, but protocols for consistent baseline energy model input have been developed for several versions of ASHRAE 90.1 and are published in the COMNET Modeling Guidelines & Procedures.

Designers may consider using energy modeling software for heating and cooling load calculations. Whole building analysis tools use a variety of load calculation methodologies and may include design day calculations and automated equipment sizing. Older software such as eQUEST uses a form of the transfer function method with weighting factors for load calculations, while EnergyPlus uses the more current heat balance method recommended by ASHRAE. Specifying design day information in eQUEST is limited to single heating and cooling design days, whereas EnergyPlus allows monthly design days to be input. It is important that the energy modeler understands which methodology is appropriate for the application and specifies the correct inputs as certain system types may dictate which load calculation method is more appropriate, such as in the case of radiant systems versus all air systems.

Capitalizing on the model investment

Energy modeling in the design process has traditionally been used to answer such routine "either/or" questions as which glass type or HVAC system should be used. These are important considerations and this is a valuable use of the tool, but the additional value that can be extracted from the energy model merits consideration because the investment is already being made. Integrated design, energy audits/retrofits, and building performance tuning are three significant areas of opportunity. The first allows for enhanced decision making during the design process and the second two both help to deliver on the good intentions of the owner and the design team.

The use of modeling to facilitate the interplay of architects’ and engineers’ design options and decisions represents great forward progress relative to the early design concept and strategy considerations of the project team. However, as shown in Table 1, the project team can capitalize on additional opportunities using the energy model from early through late stages of project progression as well as post-occupancy.

Two of the most significant impediments to such enhanced use of the energy model are often fee and response time. Fees for modeling-at least in initial project conversations-still tend to reflect the value proposition and perspective of a decade ago. Reasons for this range from the topic being completely new to some owners to an outdated belief in a prescription path of code compliance no longer viable with advancing codes due to not having recently built a building. Others see modeling as only a documentation tool, and still others see the value only in early comparisons. With the increasing number of practitioners both internal and external to AEC firms, a slow shift is taking place toward greater recognition of value and associated effort to deliver the value.

Value can fall short if the modeling struggles to keep pace with project decisions and milestones, however. Considering the dramatic improvements in software capabilities and calculation speeds, communication is now frequently the largest impediment, which can be particularly pronounced when modeling takes place external to the core AEC team. Latency in the process of sharing information externally, whether between architect and engineer or between the AEC firms and the modeler, is common. Communication can even be hampered when modeling is internal; in all cases a nimbleness with tools and effective communication are essential to keep the modeling on pace with the design process.


The building information modeling (BIM) industry has begun to embrace energy modeling as another component to BIM tools. Both Autodesk and Bentley have acquired energy analysis software companies and integrated energy modeling tools into their product offerings. Historically, the physical geometry for the model description had to be generated directly in the energy modeling software as with early versions of DOE2.1. The ability to import 2-D CAD files is now generally widespread among analysis tools. The import of 3-D model descriptions along with building material properties, internal load inputs, and schedules is also possible with Autodesk’s Green Building Studio and Bentley’s AECOSim, but with varying degrees of success based on the original BIM setup and creation approach. While BIM may be valuable at later stages of the design when model development has progressed, many architects are not using BIM tools at early phases of design when it is most beneficial to begin energy analysis.

The generation of the energy model from the BIM often requires both BIM efforts beyond traditional design workflows and additional energy model efforts to clean and debug the inputs. Industry Foundation Classes (IFC) and Green Building XML (gbXML) are the two primary data exchangeable schema between BIM and building energy simulation programs. IFC supports bi-directional transfer of building geometry and construction data information, but currently doesn’t support the exchange of HVAC system information. The gbXML schema is simpler, focused on information necessary for engineering analysis, but currently only supports one directional exchange of data between CAD and energy modeling software. Both schema currently necessitate the use of middleware software to import the BIM into most modeling software. Trace 700 and IES-VE modeling software are capable of directly importing gbXML. Middlewares such as Solibri Model Checker (SMC), Green Building Studio (GBS), Ecotect, OpenStudio Application Suite (EnergyPlus GUI), and Simergy (EnergyPlus GUI) help not only in transforming the data, but also in visualizing the geometry.

Measurement and verification

The value of energy modeling does not end at the completion of building design and construction. Once a building is occupied, energy analysis can assist in ensuring that actual building performance meets the intended design performance through the measurement and verification (M&V) process. During the construction administration phase of a project, submittal review, field inspections, start-up, and commissioning are all employed to achieve a finished product that meets the design intent. Changes may have occurred during construction-controls may not be tuned correctly, operational changes may have been made after start-up, building and equipment use may vary from original intentions-all leading to energy performance not meeting project goals. By measuring energy use and trending such operation data as setpoints and equipment use, building owners can compare this information to energy model results to help verify that building system performance is as predicted, identify causes for performance variations, and make necessary corrections or adjustments.

This process begins with updating the design model with any changes made during construction and aligning operational schedules and setpoints with actual control parameters established at start-up and commissioning. This model updating can be part of the commissioning process, and while this is occurring, measured data collection starts and continues to the end of a predetermined measurement period, usually a minimum of 1 year after occupancy. Regular monitoring of measured data should occur to ensure data quality and consistency are maintained. In addition, the data can be used in the commissioning process and can often identify problems early. Any operational changes that impact energy use, such as occupancy rate changes, should be noted for use during energy model calibration.

Energy model calibration is performed after the measurement period and initial model tuning is complete. This process changes model input parameters to those documented during the measurement period, including climate data, in order to calibrate simulated energy use with actual measured energy use. When significant variations (>5%) are discovered, the energy model can be used to identify potential causes by testing different inputs. A similar process is used on existing buildings to identify operational deficiencies and upgrade strategies that could be implemented to improve performance.

For one institutional client’s project, the M&V process identified significant variations between predicted and measured performance. Some of these variations were associated with higher than anticipated plug loads, variable air volume turndown, and exhaust fan operation. Figure 5 demonstrates differences between the measured and predicted electrical energy data with calibration steps and final error ranges.

Ongoing commissioning

Once a calibrated model is available, it offers continuing value for facility management as an ongoing commissioning tool. Buildings are complex systems requiring diligent attention to maintain energy performance. Operationally, multiple participants are responsible for servicing, monitoring, and maintaining building systems and, over time, building energy use tends to increase. Numerous studies have demonstrated the value of retro-commissioning, and energy savings of 5% to 15% have been documented (A Retrocommissioning Guide for Building Owners, PECI and EPA, 2007). Building managers can use a calibrated building model as a diagnostic tool to sustain building performance over the building’s lifecycle. The model can be integrated into building energy management systems, notifying operators when energy performance is outside of predicted usage ranges.

Paul Erickson is sustainable practice leader at Affiliated Engineers Inc., where he champions integrated design on an array of project types using performance modeling tools to help guide exploration and decision making. Bill Talbert is sustainable department facilitator at Affiliated Engineers Inc. He leads the firm’s building performance modeling team and is a member of the ASHRAE Standard 90.1 SSPC Energy Cost Budget Subcommittee.